Stainless Steel Tube for the Food Industry

Topics Covered

Introduction
Specification Comparison
     Material
     Manufacture
     Dimensional Tolerances
          Wall Thickness
          Outside Diameter
     Surface Finish
     Weld Bead
     Heat Treatment
     Mechanical Properties
     Non-Destructive Inspection
Which Specification

Introduction

This article provides the alternative specifications for the Australian food industry service. Such specifications include:

ASTM A554 “Specification for welded stainless steel mechanical tubing”

ASTM A270 “Specification for seamless and welded austenitic stainless steel sanitary tubing”

ASTM A269 “Specification for seamless and welded austenitic stainless steel tubing for general service”

ASTM A249 “Specification for welded austenitic steel boiler, super heater, heat exchanger, and condenser tubes”

AS 1528.1 “Specification for tubes (stainless steel) for the food industry”

AS1528 was revised in 2001 by key stakeholders in the Australian food manufacturing and tube industries. AS 1528 is unique, in that it covers all the associated fittings in addition to the tube.

Specification Comparison

Material

All specifications pertain to common grades, such as grades 304, 304L, 316 and 316L. AS1528.1 covers all grades of duplex and austenitic stainless steel listed in ASTM A240.

Manufacture

All specifications need fusion-welded products without filler metals. Specifications such as ASTM A270, ASTM A269 and AS 1528 also cover seamless products.

Dimensional Tolerances

Wall Thickness

ASTM A554 requires ±10% of nominal - no nominal thicknesses are stipulated.

ASTM A270 requires ±12.5% of nominal - no nominal thicknesses are stipulated.

ASTM A269 requires ±10% of nominal for sizes over ½" - no nominal thicknesses are stipulated.

ASTM A249 requires ±10% of nominal - no nominal thicknesses are stipulated.

AS 1528 specifies nominal thicknesses of 1.6mm for all outside diameters, (ODs) except 2 mm for 203.2 mm OD; other thicknesses can be specified by purchasers. Standard tolerance is +nil, -0.10mm. The all-minus tolerance recognizes the usual practice for tube, to all specifications, to be produced towards the lower limit of the tolerance range. A range of between 1.52 and 1.58 mm is typical. This tolerance also applies to tube fittings.

Outside Diameter

Table 1. Specification requirements for standard inch series OD tube sizes

* ASTM A554 tolerances for the weld bead removed condition.
* AS1528 also covers OD sizes 12.7, 19.0, 31.8, 127.0, 152.4 and 203.2mm

All these tube specifications provide limits for wall thickness and OD. The inside diameters are not mentioned separately.

Surface Finish

The surface finish properties of various specifications, recommended for Australian food industry services, are as follows:

• ASTM A249 and ASTM A269 require surfaces that are free of scales and rust. Annealing of the tube is usually carried out in a controlled environment, and this “bright annealed” finish is considered acceptable.
• ASTM A270 needs selection of both external and internal surfaces. The possible conditions include mill finish, abrasive polishing with 80, 120, 180 or 240 grit, special polishing or electropolishing. Surface finishes can be specified in terms of Ra values without any limits.
• ASTM A554 specifies “direct off mill” or “free of scale" finish as standard. Special finishes, if required, need to be mentioned in the order. Thus a large quantity of A554 tube is supplied in buffed or externally polished conditions.
• AS1528 covers the external surface "buff polished" or "as produced". The internal surface needs to be 2B finish, quoted as typically 0.3µm Ra. Studies suggest that the typical roughness ranges from 0.10 to 0.20 µm Ra for 1.6mm 2B coil. Precautions must be taken to prevent significant degradation of roughness during the manufacture of tube.

Weld Bead

Listed below are the weld bead procedures for different steel grades:

The handling products used in the food industry require a tube without weld bead remnant on the inner surface.

• ASTM A249 requires the weld to be cold-worked after welding, and before final heat treatment.
• ASTM A269 does not require any cold-working or weld bead control.
• ASTM A270 does not require weld bead.
• ASTM A554 can be supplied with the weld bead left on, and hence it complies with the "Bead Removed" option of A554.
• AS1528 requires removal of the weld bead. There is also a requirement that the internal surface needs to be smooth, with no lack of weld penetration and no crevices adjacent to welds.

Heat Treatment

The following stainless steel grades can be heat-treated:

• ASTM A249, ASTM A269 and ASTM A27 specify that all materials can be furnished in the heat-treated condition. Heat-treatment usually involves annealing methods, such as solution annealing or solution treatment. In practice, heat-treatment is not a basic requirement for food industry applications.
• ASTM A554 is usually supplied in “as welded” condition, i.e. no heat-treatment required after tube forming.
• AS1528 allows the purchaser to specify either annealed or un-annealed conditions of steel products.

Mechanical Properties

The mechanical properties of various stainless steel grades, commonly used in food industries, are given below:

• ASTM A249 requires extensive mechanical testing for use in critical environments in boilers or heat exchangers.
• ASTM A269 requires reverse flattening, plus flange and hardness tests. It does not require tensile testing.
• ASTM A270 requires a reverse flattening test only.
• ASTM A554 does not require mechanical testing as standard.
• AS1528 requires the tube to be made of strips that comply with ASTM A240. It does not require tensile or hardness testing.

Non-Destructive Inspection

The non-destructive inspection procedures for different steel grades are listed below:

• ASTM A249, ASTM A269, ASTM A270 and AS1528.1 all require 100% hydrostatic or eddy current testing.
• ASTM A554 includes a supplementary requirement that deals with the possibility of non-destructive testing. However, this is applicable for ASTM A554 tube.

Which Specification

The following section provides a summary on each specification employed in food industries:

• ASTM A249 specifies weld bead removal. This requirement can be met from other standards. ASTM A249 does not require annealing in most food applications.
• ASTM A269 again requires annealed tube. Conversely, it does not specify internal weld bead removal, which generally is a food industry requirement. ASTM A269 is a stock item, and it proves uncompetitive against un-annealed tube.
• ASTM A270 also requires the tube in the annealed condition, and it describes nothing about weld bead. The finish options available in this specification are very comprehensive.
• ASTM A554 is intended for mechanical applications, and not for pressure containment or sanitary use.
• AS1528 is the safest and most cost-effective option. It is specifically employed in food industries, as it specifies the features necessary to achieve high integrity lines for hygienic applications without requiring expensive mechanical testing. Annealing can be done if required, and surface finishes can be further specified. Batch traceability marking, used to verify many food and pharmaceutical plants, is necessary. Another key benefit is the presence of matching specifications for tube fittings.